A shock is likely to occur as a direct-acting hydraulic cylinder (lift cylinder) disposed at a forklift and used to lift a load becomes fully extended and stops. The shock is usually damped with a cushion pressure imparted near the end of an extending stroke.
This type of hydraulic cylinder includes a piston seal (seal ring) disposed on the outer circumference of a piston. The piston seal partitions a cushion pressure chamber so as to allow a hydraulic fluid to be collected into the cushion pressure chamber in a quantity required to create the cushion pressure.
The following is a description of the structures adopted in piston seals included in this type of direct-acting hydraulic cylinder in the related art.
The piston seal disclosed in JP 2000-2207A includes a seal ring that slidably contacts an inner circumferential surface of a cylinder tube and an O-ring that presses the seal ring against the inner circumferential surface of the cylinder tube.
The seal ring and the O-ring are mounted side-by-side in a housing groove located on the outer circumferential side of the piston, so as to form a straight line with respect to the radius of the piston.
When the piston moves up during an extending operation of a hydraulic cylinder, an elastic restorative force of the O-ring presses the seal ring against the inner circumferential surface of the cylinder tube and a cushion pressure is thus created.
The piston seal disclosed in JP10-122205A includes a rubber seal ring in slidable contact with an inner circumferential surface of a cylinder tube and a backup ring constituted of resin.
The seal ring and the backup ring are mounted side-by-side along the axis of the piston within a housing groove located on the outer circumferential side of the piston.
When the piston moves up during an extending operation of a hydraulic cylinder, the elastic restorative force of the rubber seal ring causes it to be pressed against the inner circumferential surface of the cylinder tube and a cushion pressure is thus created.
At this time, the backup ring comes into contact with the seal ring and thus prevents the rubber seal ring from being pulled out of the housing groove.